The proposed study investigates an efferent inhibitory circuit of the central auditory system. In both birds and mammals, the neural centers involved in sound localization receive inhibitory efferents from higher order nuclei. In birds, inhibitory feedback to the cochlear nuclei and binaural NL arise from a distinct pair of nuclei residing just ventral to the second and third order nuclei. This elegantly arranged system in birds provides a unique opportunity to gain a mechanistic and holistic understanding of the role of inhibitory efferents in a functionally well defined circuit of the auditory system. The long term goal of the study is to further our understanding of the contribution of efferent feedback to sound localization. The specific aims are: 1) Evaluation of the contributions of efferent inhibition to response properties in the avian cochlear nucleus; 2) Evaluation of response properties and classification of SON neuron populations; 3) Determine the role of the commissural connection of the SONs. The models that will be tested in this proposal involve the role of this inhibitory circuit for equalizing afferent inputs for binaural computations. The requirement for this equalization process has been proposed in models of binaural function that date to the 1970s, but the presence of such a system has not been demonstrated. The findings of the proposed studies could have direct implications for a number of hearing disorders, in particular, profoundly deaf patients whose treatment includes bilateral auditory prostheses. Binaural hearing provides advantages for detecting moving signals, signals in noise such as speech, and sound location compared to monaural conditions. These studies will enhance understanding of the central mechanisms of binaural processing, and this information may contribute to refinements of treatment strategies for the hearing impaired and in particular, bilaterally treated cochlear implant recipients. This proposal will investigate central mechanisms of binaural hearing. Binaural hearing is crucial for listeners to discriminate signals in noise as well as the location of sound sources. Development of modern auditory prosthetic devices relies on an understanding of auditory brain function. The proposed experiments will focus on revealing principles of binaural computations in the auditory system.